This application requests funding for the study of single isolated cardiac cells, using electrophysiological, pharmacological and radionuclide tracer techniques. Within the last year, my colleagues and I have perfected methodology for isolating and recording intracellularly from single cardiac cells derived from the bullfrog atrium. Our preliminary experiments indicate that these cells (i) have normal resting and action potentials, (ii) are responsive to adrenaline, acetylcholine, and ouabain, and (iii) have relatively long space constants, making them suitable for voltage clamp experiments. The major experimental goals of the proposed experiments will be: (i) to utilize a two microelectrode method of voltage clamping in order to analyze quantitatively the slow inward current isi and the outward potassium current(s), (ii) to use the information obtained in (i) to study in detail the mechanism of repolarization as well as the mode(s) of action of acetylcholine, adernergic agonists, cardiac glycosides, and anti-arrhythmic agents. In parallel experiments three developmental projects will be undertaken: (i) The methods for measuring 42K and 36C1 transport in isolated bullfrog atrial cells will be assessed. (ii) Attempts will be made to apply the 'patch clamp' technique of Neher and Sakmann to the single atrial cell. This technique may provide a means of studying the biophysics of muscarninic acetylcholine receptors. (iii) An effort will be made to develop enzymatic dispersion techniques for the isolation of single mammalian ventricular and/or Purkinje cells. Success in the proposed voltage clamp experiments will significantly advance present understanding of the ionic basis for the electrophysiological events in atrial muscle. The developmental projects will provide the necessary data for setting the direction and emphasis of my research in the short-term future.